The conventional eddy-current brakes had the defect that their weight per braking torque developed was large in comparison with the brakes of other systems. FIG. 11 of the accompanying drawings illustrates in section a typical example of conventional disc type eddy-current brakes. In this brake, as shown in the drawing, flange couplings 1, 2 are joined to the axle to be braked. Main shaft 3 is joined to said couplings, 1, 2 so that they won't rotate relative to each other but will be able to rotate integrally with the axle to be braked. Also, discs 4, 5 are fixed to said main shaft 3 so that they won't rotate relative to each other. Electromagnets 10, each comprising a coil 6 and magnetic pole 7, are provided radially on the same circumference of a circle centered by main shaft 3. Said electromagnets 10 are fixed to a fixing plate 9 in a stelliform arrangement with a slight spacing from said discs 4, 5. Main shaft 3 is also rotatably secured to said fixing plate 9 through ball bearings 8. This fixing plate 9 itself is fixed immovably to a brake block. Thus, normally, discs 4, 5 rotate together with the axle to be braked. In this state, electromagnets 10 are not electrified. They are electrified when the axle is to be braked. The magnetic line of force generated by each said electromagnet 10 forms a closed curve involving magnetic pole 7 and disc 4 or 5, and an eddy current is generated in said discs 4 and 5 to produce a force acting oppositely to the direction of rotation, viz. braking force, on said discs 4 and 5.
Thus, in the eddy-current brakes, rotors made of a ferromagnetic material and magnets are provided in such an arrangement that each rotor crosses the magnetic flux and a braking force will be produced by an eddy current generated in said rotor. In this system, therefore, the smaller the electric resistance of the material of the rotors, i.e., discs 4, 5 in FIG. 11, the greater becomes the braking torque developed provided that the other conditions, namely wire size and number of turns of coil 6, electric current applied thereto and space between magnetic pole 7 and disc 4 (5) are the same. However, as it is an essential codition that discs 4, 5 be made of a magnetic material, no remarkable difference is produced in the torque developed no matter what type of magnetic material is used. Thus, it has been the common problem to the conventional eddy-current brakes that there can not be obtained a large braking torque in comparison with the brakes of other systems no matter what material is used for the rotors, viz. discs 4, 5. Also, in the conventional eddy-current brake illustrated in FIG. 11, each electromagnet 10 is constituted by a magnetic pole 7 projecting axially from the center of bottom plate 11 and a coil 6 comprising a copper wire wound up on said magnetic pole 7, and such electromagnets 10 are disposed opposing to and slightly spaced-apart from discs 4, 5 so that the opposite polarities are positioned alternately to each other. Therefore, the magnetic line of force generated by each said electromagnet 10 describes a closed curve that passes magnetic pole 7 of the adjacent electromagnets 10, fixing plate 9 and one of the discs 4 or 5. Thus, because of the long magnetic path, the intensity of the magnetic field formed is weak.
The object of this invention is to provide an eddy-current brake which is free of said problems of the prior art and capable of developing a large braking torque per unit weight.